ABSTRACT- The mechanisms of radiation-induced bystander effects remain largely unknown. Bystander responses are thought to depend on activation of cellular communication processes. Recent studies have speculated that several crucial signaling transduction pathways could play a major role in bystander effects. These crucial signal transduction pathways are controlled by a coordinated regulation of several key molecular mediators. In this study we examine one such mediator of cellular communication, the pro-inflammatory cytokine tumor necrosis factor a (TNF-a). To better understand the involvement of TNF-a in bystander effects, we proposed to track the distribution, releasing dynamics and transport of TNF-? in real-time using the nanoparticle probes before and after low dose LET radiation exposures. First, to develop the nanoparticle sensors for real-time detection of TNF-a on living cell surface, we used two types of nanoparticle probes: (i) 20 nm fluorescence nanospheres and (ii) silver (Ag) nanoparticles. Next, to study whether the antibody-linked nanoparticle probes are still able to preserve the biological function of the antibody, we used IgG to link with nanoparticles and study its biological activities using protein A. The results demonstrated that (i) the IgG-linked nanoparticles were able to recognize protein A molecules in the solution, (ii) nanoparticle-linked IgG molecules still preserve the biological activities of IgG and can serve as a nanosensor to recognize the protein A, and (iii) unlike fluorescence probes, these nanoparticles resist to photodecomposition, allowing us to monitor the motion of individual signaling molecules for extended hours. Following similar scheme we conjugated TNF-a antibody with nanoparticle probes and used those nanoprobes to follow TNF-a molecules releasing from irradiated living cells in real-time. The results provide evidence that TNF-a could act as a bystander mediator by traveling from the irradiated cells to unirradiated cells. This research was supported by the Office of Science (BER), U.S. Department of Energy, Grant No. DE-FG02-03ER63646.